Tuesday, December 27, 2016

How Did a Chunk of India and Eurasia Just Disappear?

Case of Earth's missing continental crust solvedHalf
of the mass of Eurasia and India is missing, new research finds, and
may have been swallowed up by the Earth's mantle. How do you make half
the mass of two continents disappear? To answer that question, you first
need to discover that it's missing.

That's what a trio of University of Chicago geoscientists and their
collaborator did, and their explanation for where the mass went
significantly changes prevailing ideas about what can happen when
continents collide. It also has important implications for our
understanding of when the continents grew to their present size and how
the chemistry of Earth's interior has evolved.

The study, published in Nature Geoscience, examines the collision of
Eurasia and India, which began about 60 million years ago, created the
Himalayas and is still in (slow) progress. The scientists computed with
unprecedented precision the amount of landmass, or "continental crust,"
before and after the collision.

"What we found is that half of the mass that was there 60 million years
ago is missing from Earth's surface today," said Miquela Ingalls, a
graduate student in geophysical sciences who led the project as part of
her doctoral work.

The result was unexpectedly large. After considering all other ways the
mass might be accounted for, the researchers concluded that so huge a
mass discrepancy could only be explained if the missing chunk had gone
back down into Earth's mantle -- something geoscientists had considered
more or less impossible on such a scale.

When tectonic plates come together,
something has to give. According to plate tectonic theory, the surface
of Earth comprises a mosaic of about a dozen rigid plates in relative
motion. These plates move atop the upper mantle, and plates topped with
thicker, more buoyant continental crust ride higher than those topped
with thinner oceanic crust.

Oceanic crust can dip and slide into the mantle, where it eventually
mixes together with the mantle material. But continental crust like that
involved in the Eurasia-India collision is less dense, and geologists
have long believed that when it meets the mantle, it is pushed back up
like a beach ball in water, never mixing back in.

Geology 101 miscreant

"We're taught in Geology 101 that continental crust is buoyant and can't
descend into the mantle," Ingalls said. The new results throw that idea
out the window.

"We really have significant amounts of crust that have disappeared from
the crustal reservoir, and the only place that it can go is into the
mantle," said David Rowley, a professor in geophysical sciences who is
one of Ingalls' advisors and a collaborator on the project. "It used to
be thought that the mantle and the crust interacted only in a relatively
minor way. This work suggests that, at least in certain circumstances,
that's not true."

The scientists' conclusion arose out of meticulous calculations of the
amount of mass there before and after the collision, and a careful
accounting of all possible ways it could have been distributed.
Computing the amount of crust "before" is a contentious problem
involving careful dating of the ages of strata and reconstructions of
past plate positions, Ingalls said. Previous workers have done similar
calculations but have often tried to force the "before" and "after"
numbers to balance, "trying to make the system match up with what we
think we already know about how tectonics works."

Ingalls and collaborators made no such assumptions. They used recently
revised estimates about plate movements to figure out how large the two
plates were at the onset of collision, and synthesized more than 20
years' worth of data on the geology of various regions of Earth to
calculate how thick the crust would have been.

"By looking at all of the relevant data sets, we've been able to say
what the mass of the crust was at the beginning of collision," Rowley
said.

Limited options

There were only a few places for the displaced crust to go after the
collision: Some was thrust upward, forming the Himalayas, some was
eroded and deposited as enormous sedimentary deposits in the oceans, and
some was squeezed out the sides of the colliding plates, forming
Southeast Asia.

"But accounting for all of these different types of mass loss, we still
find that half of the continental crust involved in this collision is
missing today," Ingalls said. "If we've accounted for all possible
solutions at the surface, it means the remaining mass must have been
recycled wholesale into the mantle."

If large areas of continental crust are recycled back into the mantle,
scientists can at last explain some previously puzzling geochemistry.
Elements including lead and uranium are periodically erupted from the
mantle through volcanic activity. Such elements are relatively abundant
in continental crust, but scarce in the mantle. Yet the composition of
some mantle-derived rocks indicates that they have been contaminated by
continental crust. So how did continental material mix back into the
mantle?

"The implication of our work is that, if we're seeing the India-Asia
collision system as an ongoing process over Earth's history, there has
been a continuous mixing of the continental crustal elements back into
the mantle," said Rowley. "And they can then be re-extracted and seen in
some of those volcanic materials that come out of the mantle today."